Trigger on Previous button

I'm working with the Trial version of Storyline at the moment (and thoroughly enjoying the experience, I might add!).

My query is as follows:

I have a quiz with two custom-made screens for correct and incorrect feedback. Once the feedback (either correct or incorrect) appears and I select Next, I am taken to the next screen in the sequence (Screen 4, for argument's sake).

Currently, if I select the Previous button on Screen 4, I am taken back to whichever feedback screen I visited, and then a second Previous takes me on back to my quiz.

I want the Previous button on Screen 4 to take me straight back to my quiz - but when I set up a trigger, the Previous button does not appear as an object in the trigger wizard.